Clarify some documentation and POD

I was reading over these comments & POD and found some of it to be
lacking.
Fix a long-standing bug with "call" not being able to call modules that use our constant exporter

See the Changes and comment I've added to Lazy.pm for details. This
fixes the bug noted in the main.git repo's 887981b at work, which I've
distilled down to a testcase here.

TODO: There's no test that fails if I remove the {$pkg_importer} from
the nested hashes I'm creating here.
Add a new test to see if we're actually defining constants
tests: Actually test if the "override" non-exporting works properly
We now use FATAL "recursion" warnings in Constant::Export::Lazy
Don't die when someone accesses a non-existent stash (yet again)

This comment hasn't made sense since 412a622, fix that up.
Get rid of the Constant::Export::Lazy::Ctx::new() method

I was thinking of just getting rid of that package entirely, but I like
the clean-ness of not having $ctx->import() or even
$ctx->_normalize_arguments() be possible.